It is commonly known that certain electrical characteristics of semiconductor devices vary under the influence of both internal and external factors. For instance, the beta of a bipolar transistor varies according to variables in its manufacturing Process, to the applied supply Voltage, and to the environment Temperature. These factors are referred to by the acronym PVT.
In many applications of semiconductors, a large change in the electrical characteristics is undesirable and efforts are made to reduce the effects of PVT factors. A common technique--inverse feedback--senses a change of operation (e.g., signal delay), and then takes some action to reduce the change. Another approach is to sense the magnitude of PVT factors and then to modify some aspect of the circuit in a predetermined manner, thereby compensating the change of characteristics.
The present invention arose in the context of large-scale digital ICs (LSIs) used in printer controllers. A typical LSI device contains hundreds of thousands of transistors arranged in many different internal logic structures. It also contains several (relatively) large transistors as output amplifiers which are needed to drive the LSI bonding pads and whatever external loads are connected to them. As the operating speed of LSI devices has inexorably increased, a couple of PVT problems related to the output amplifiers have become serious enough to warrant a lot of effort towards their solution. These problems are a) change in the amplifier signal time delay and b) change in the amplifier signal risetime. In this disclosure, "risetime" refers both to the total transition time and to how abruptly the signal begins the transition.
Change in the amplifier signal time delay can cause logic failures in a printer control system due to excessive skew among various signals in the system. For instance, if the system clock is 50 MHz, a half-cycle (10 nanosecond) change in delay of a signal can cause a register to store wrong information. Delay variations of this order are not uncommon over the range of PVT factors.
Likewise, if the signal risetime becomes much faster than the designed value, electromagnetic interference can be radiated from signal lines connected to the LSI device. And, if ringing accompanies the faster risetime, there may be logic errors because of multiple threshold crossings.
If the amplifier gain is changed, this also varies signal time delay and risetime. However, this effect provides an opportunity for compensating PVT-caused changes in these characteristics. FIG. 1 illustrates one of several prior-art techniques for such compensation. Output amplifier 12 is located within IC 10, and is connected to an external load represented by the RC combination 22. Also within IC 10 is a resistor 16, whose value is affected by the same PVT factors that affect amplifier 12. Resistor 16 is, typically, either an isolated, doped channel in the substrate or a MOSFET operated below its pinch-off region. It is connected to an internal bias voltage V, and is paired with a stable external resistor 18 to create a voltage divider whose output is connected to a controller 20. The function of controller 20 is to convert the voltage divider output into an appropriate control signal on line 14 to apply to vary the gain of amplifier 12. Typically, amplifier 12 can be modelled as a transconductance, and its gain is varied by adjusting the coefficient of its dependent current source. Hence, controller 20 can be regarded as a translation circuit, whose function is to relate the PVT factors (represented by the voltage divider output) to the appropriate amplifier gain required to keep its delay and risetime constant. Now, the amplifier delay and risetime are affected not only by its gain and the PVT factors, but also by its load. If the admittance of load 22 were increased to, say, twice its nominal value, both delay and risetime would increase substantially. (This might happen if the amplifier drove a memory bus line, and optional memory packages were added.) If this happens, the compensation will be inaccurate unless controller 20 is modified to accomodate the changed conditions.
In LSI applications, such as printer controllers, it is common to have output amplifiers of different sizes driving loads of different values. With the design illustrated in FIG. 1, to get accurate compensation it would be necessary to have as many individual controllers as there are combinations of amplifiers and loads.
Clearly, what is needed is a device and technique for keeping amplifier delay and risetime substantially constant, and for accomodating manufacturing differences, environmental changes, and configuration changes with neither undue complexity nor unacceptable inaccuracy.